1. Field of the Invention
The invention is directed to an amorphous alloy for strip-shaped sensor elements having low saturation induction for employment in anti-theft labels, magnetic field detectors or the like.
2. Description of the Prior Art
Thin strips of a material having a very low retentivity are required for anti-theft labels Commercially available strips of both crystalline and amorphous material have been employed for this purpose. The standard dimensions for such strips are a ribbon width of less than 3 mm, a ribbon thickness of less than 40 .mu.m, and a label length of 50-100 mm, or below in individual cases. Important for the functioning of such strips is that the material can be completely magnetized, or remagnetized with optimally low exciting magnetic fields. As a result of the non-linearity of the magnetization curve of the strip when the magnetic saturation is reached, then upper harmonics (for example) of the excitation frequency are generated in a corresponding receiver coil of an anti-theft system given re-magnetization, these upper harmonics serving the purpose of detecting the strip, and thus a possible theft.
That field strength H.sub.s needed for completely magnetizing the strip is essentially determined by the geometry of the strip (magnetic shearing effect) and by the magnetic anisotropy energy transversely relative to the strip direction. The following relation is valid in strip direction: ##EQU1## wherein w denotes the width, t.sup.l denotes the thickness, l denotes the length of the strip, B.sub.s denotes the saturation induction and H.sub.A denotes the magnetic anisotropy field. The factor a is likewise dependent on the strip geometry, though only to a slight degree, and can be essentially considered to be a constant.
In order to arrive at a detectable, significant signal, the magnetic excitation field strength in the customary systems must be roughly on the order of magnitude of, or greater than, the saturation field strength H.sub.s insofar as possible. The excitation field strength can not, however, be excessively high for several reasons, for example, to avoid false alarms due to other ferro-magnetic articles, for reasons of power consumption for the excitation field strength, for reducing unnecessary losses, or for heating.
Similar conditions are frequently present in magnetic field sensors for the acquisition of magnetic fields as well. The sensitivity of these sensors generally increases with increasing strip length, wherein a uniformity of the aforementioned equation is also critical.
The demagnetizing field is noticeably diminished in the strip direction according to the above equation on the basis of the specific selection of the strip geometry, i e. low width and thickness and relatively long label length This has the desired effect that the magnetic strip can be re-magnetized in relatively low excitation fields, and thus supplies the desired signal.
The saturation field strength H.sub.s reduced even more by specific heat treatments, which cause the anistropy field H.sub.A to nearly disappear. This, for example, is the case for magnet material having an intrinsically rectangular magnetication loop, for which reason such a material has proven especially suitable in many cases.
The optimization of the magnetic strips for anti-theft labels hitherto ensued by adapting the geometry and by heat treatment of commercially available magnetic material, whereby the heat treatment ensues in the magnetic field parallel to the longitudinal axis of the band.
Problems, however, arise when the available space and, thus, the strip length l is limited for spatial reasons (for example, miniaturization). In order to nonetheless obtain a low shearing field in such cases, w.multidot.t.multidot.B.sub.s (cf. the equation) must be correspondingly reduced. This can be achieved to a certain degree by reducing width w and thickness t. Given extremely small widths and thicknesses, however, increasing problems arise in the manufacture and manipulation of ribbon (or of wire) having such a slight cross-section.